Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have been proposed for use in electrical vehicle power plants to replace internal combustion engines. A solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and air or oxygen (O2) is supplied to the cathode.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+) and electrons (e−) to form water (H2O).
The main function of a fuel processor in the fuel cell system is to provide a controlled hydrogen-containing stream to the fuel cell stack. The fuel cell stack converts the chemical energy in the hydrogen to electrical power to charge capacitors or batteries or to directly power a device such as an electric motor. In hybrid applications, a storage medium such as capacitors or batteries removes some of the problems that are associated with transient demand. For non-hybrid applications, and to a lesser extent in hybrid applications, the fuel processors need to provide a dynamic flow rate of hydrogen-containing gas to the fuel cell stack. When a device is directly powered by the fuel cell, the amount of hydrogen that is required is determined by the demand for power output from the fuel cell. For example in automotive applications, the driver demands power by depressing the accelerator pedal. Acceleration requires the electric motor to turn faster, which requires more current. When the accelerator is depressed, the fuel processor increases the hydrogen that is provided to the fuel cell. The current output by the fuel cell increases and the electric motor accelerates the vehicle.
The fuel processor produces a reformate stream that is composed primarily of hydrogen, carbon dioxide, nitrogen, water, methane and trace amounts of carbon monoxide. During operation, the fuel processor provides the flow rate of hydrogen that is required to meet the current demand for power. As can be appreciated, the demand for power can vary significantly. For example, a vehicle moving in rush hour traffic may repeatedly require sudden acceleration followed by deceleration or braking. Thus, the delivery of hydrogen to the fuel cell stack must vary accordingly. Fuel processors may also require careful metering of air and fuel to maintain precise oxygen to carbon ratio control.
Additionally, a typical fuel processor may use an autothermal reforming reactor as a primary reactor to initiate the production of the hydrogen-containing reformate stream. Autothermal reforming reactors introduce reactants (fuel, oxidants, steam, etc.) into the front of the reactor and allow the associated reactions to occur to completion as the reactants flow through the reactor. The fuel can come in a variety of forms, such as methanol, gasoline, ethanol, etc. The oxidant is typically provided in the form of oxygen (O2) or air (O2 mixed with N2). The steam is typically superheated steam which supplies heat and water to the reactor. An autothermal reforming reactor is capable of converting the fuel into a nitrogen/steam diluted reformate stream containing hydrogen and carbon oxides that result from the combined partial oxidation reaction and steam reforming reaction, the extent of each being dependant on the operating conditions (e.g., availability of an oxidant and/or steam and temperature of the reactor). A steam reformer may also serve as the primary reactor which eliminates the nitrogen diluent that is present when partial oxidation is also included as in autothermal reforming. These two different reactions differ in their efficiencies, the operating conditions that increase and/or maximize the efficiencies, and their ability to quickly adjust to transient changes in the demand for the hydrogen-containing reformate stream. For example, the steam reforming reaction is typically more efficient at producing the hydrogen-containing reformate stream than the partial oxidation reaction. Additionally, the steam reforming reaction is more efficient at higher pressures (5-7 bars). The partial oxidation reaction is able to respond more quickly to transient changes in the demand for the hydrogen-containing reformate stream than the steam reforming reaction. Transient response requires rapid response and control in fuel, air and steam delivery, but the rates of response may vary. Furthermore, all the reactors downstream of the primary reactor must also be able to respond rapidly as well.
While the fuel cell stack can consume as much hydrogen as it needs based on the electrical load applied to the fuel cell stack, mismatching the hydrogen flow and the electrical load is problematic. An under-fueled stack may cause some of the fuel cells to temporarily have reverse polarity, which may damage the fuel cell stack. An over-fueled stack will not damage the fuel cell stack but will increase the H2 exhausted. If the exhausted hydrogen is fed to a combustor, for example, increased combustion temperature may damage the combustor or cause NOx emissions to increase if additional air control is not used.
Therefore, what is needed is a fuel processor that can provide a required flow rate of hydrogen-containing reformate and respond quickly to transient changes in the demand for the hydrogen-containing reformate. Additionally, it is advantageous to provide these capabilities in an efficient manner.